The present invention relates to the field of devices and methods for assisting hearing in persons and particularly to the field of transducers for producing vibrations in the inner ear.
The seemingly simple act of hearing can easily be taken for granted. Although it seems to us as humans we exert no effort to hear the sounds around us, from a physiologic standpoint, hearing is an awesome undertaking. The hearing mechanism is a complex system of levers, membranes, fluid reservoirs, neurons and hair cells which must all work together in order to deliver nervous stimuli to the brain where this information is compiled into the higher level perception we think of as sound.
In most standard texts on hearing, it has been generally reported that the upper limit of normal hearing is about 20,000 Hz. Nonetheless, since the 1950s, scientists have studied the use of high frequency applications for use with hearing impaired individuals. Surprisingly, bone-conducted ultrasonic hearing has been found capable of supporting frequency discrimination and speech detection in normal, older hearing impaired, and profoundly deaf human subjects.
Although, the mechanism that allows humans to perceive ultrasonic stimuli is not well known or understood. There are two leading hypotheses relating to how ultrasonic perception of sound may occur. The first theory involves a hair cell region at the base of the cochlea which is believed to be capable of interpreting ultrasonic signals. The second theory involves the vestibular and saccular regions that may also be capable of responding to ultrasonic stimuli. Unfortunately, the anatomy of the ear (the tympanic membrane and ossicles) is unable to deliver acoustic ultrasonic energy, perceived in the environment, to either the cochlear or vestibular regions because of the impedance mismatch of the tympanic membrane.
In U.S. Pat. No. 4,982,434 to Lenhardt et al., herein incorporated by reference for all purposes, Lenhardt et al. describes a sound-bridge for transferring ultrasonic vibratory signals to the saccule via the human skull and independent of the inner ear. Because the ultrasonic vibrations are transmitted directly to the bones of the skull, frequencies are used that are perceived by the saccule and not by the inner ear. The supersonic bone conduction (ssBC) transducer, described in Lenhardt et al., is an electric to vibration type used to apply the ultrasonic signal as ultrasonic vibration to the skull, preferably at the mastoid interface. Piezoelectric transducers are typically used in ultrasonic applications due to their high impedance in the ultrasonic range.
Unfortunately, for an ultrasonic hearing device, such as the one described in Lenhardt et al. to provide acceptable fidelity, the ultrasonic vibratory signal must be placed as close as possible to the regions of the ear which have ultrasonic frequency perception capability. The piezoelectric bone conduction system described in Lenhardt et al. requires that the signal be delivered across the skin to the skull. This type of signal transfer can result in a poor or even a lost signal. Moreover, because the ultrasonic vibration must be translated to the cochlear or vestibular regions from outside the skull, there is a substantial amount of loss of the vibratory signal, and potentially a substantial amount of distortion could be introduced in the perceived signal. Although a piezoelectric vibrator may be sufficient for use with most frequency levels, it does have limitations in the ultrasonic frequency range. For example, piezoelectric devices tend to have outputs that result in highly peaked responses which may hinder speech perception in the ultrasonic condition. Because piezoelectric materials have a crystalline composition, the devices tend to be very stiff and typically resonate at frequencies of 6 kHz or higher.
In view of these limitations, an ultrasonic direct drive hearing system is desired which can be positioned as close to the inner ear fluid as possible to stimulate the inner ear fluid (or vestibule) or as close as possible to the saccule to stimulate the saccular system with an ultrasonic signal.